In the state of the art, it is known to monitor fill-level of a medium by detecting whether the conductive medium is providing electrical contact between a probe electrode and the wall of a conductive container or a second electrode. Since, in the case of many media, an accretion can form on the probe unit, so-called guard electrodes are used, which lie at the same electrical potential as the probe electrode and surround the probe electrode (see e.g. DE 32 12 434 C2). Depending on the character of the accretion, it is, however, possible, that there will be difficulties with suitably producing the guard signal.